Synapse formation is characterized by the accumulation of molecules at the site of contact between pre- and postsynaptic cells. Agrin, a protein implicated in the regulation of this process, causes the clustering of acetylcholine receptors (AChRs). Here we characterize an agrin-binding site on the surface of muscle cells, show that this site corresponds to alpha-dystroglycan, and present evidence that alpha-dystroglycan is functionally related to agrin activity. Furthermore, we demonstrate that alpha-dystroglycan and adhalin, components of the dystrophin-associated glycoprotein complex, as well as utrophin, colocalize with agrin-induced AChR clusters. Thus, agrin may function by initiating or stabilizing a synapse-specific membrane cytoskeleton that in turn serves as a scaffold upon which synaptic molecules are concentrated.